The invention pertains to stacked patch antennas. More particularly, the invention pertains to stacked patch antennas with improved frequency band isolation and multiple (greater than two) frequency bands of operation.
A patch antenna is a type of antenna that is particularly suitable for relatively narrow band operation. A patch antenna usually comprises a dielectric panel with conductive patterns or patches deposited on both sides of the dielectric panel. The top conductive pattern or patch is the radiator and is sized and shaped to resonate at a particular frequency. This top patch (hereinafter termed the radiating patch of the patch antenna) acts as a parallel plane, micro strip transmission line serving as an antenna by giving in-phase linearly or circularly polarized radiation. The radiating patch is fed, for example, by a coaxial feed. A coaxial feed comprises a central conductor encircled concentrically by a dielectric, with the dielectric encircled concentrically by another, outer conductor serving as a shield. The outer conductor typically is connected to a ground plane. The inner conductor is connected to the radiating patch. The signal, whether transmitted from the antenna or received by the antenna, travels as a voltage differential between the inner conductor and the outer, grounded conductor. The radiating patch radiates the signal from its edges. The bottom conductive pattern acts as a ground plane for the radiating patch and is hereinafter termed the ground patch of the patch antenna.
One of the fundamental advantages of patch antennas is that they are extremely compact. However, they usually radiate efficiently over only a fairly narrow bandwidth. Accordingly, they are most commonly used in narrow bandwidth applications, such as GPS (global positioning satellite) systems, which operates over one or two very narrow frequency bands.
Particularly, the GPS system operates in two distinct bandwidths, a military band at 1227 MHZ and a civilian band at 1575 MHZ. GPS receivers that are allowed to access the military bandwidth (and thus operate with much higher accuracy) actually access the signals on both bandwidths. Accordingly, such systems would require two patch antennas, each designed to resonate in one of the two frequency bands.
In the past, a known method of feeding the radiating patch is to connect the inner conductor of the coaxial feed to the patch at a natural feed point of the patch. The natural feed point of the radiating patch is the point at which it presents an apparent fifty ohm impedance when a conductor is coupled at that point. This locus of points typically is offset from the geometric center of the radiating patch.
Stacked patch antennas are known in which two patch antennas are stacked on top of each other. For sake of clarity, the following terminology will be used hereinafter in this specification. The individual antennas in a stacked patched antenna assembly will be referred to as patch antennas or simply antennas. The top conductive pattern of a patch antenna will be termed the radiating patch of the patch antenna and the bottom conductive pattern, if included, will be termed the ground patch of the patch antenna. The entire stacked patch antenna assembly comprising multiple patch antennas will be referred to as a stacked patch antenna assembly.
A stacked patch antenna assembly is suitable for the aforementioned two band GPS type application. Conventional stacked patch antenna assemblies typically have used one of two types of feed arrangements. In one arrangement, only one patch antenna is directly fed while the other is parasitically coupled to the first patch antenna. In the other type of feed arrangement, each patch antenna is directly fed. In the type of feed arrangement where each patch antenna is directly fed, each feed, which comprises a coaxial cable with an inner and an outer conductor, has the outer conductor shorted to the ground patch at some non-centered point on the patch antenna.
In both of these types of feed arrangements, the amounts of isolation achievable between the operating frequencies of the two (or more) patch antennas is quite limited. In the former type, in which one of the patch antennas is parasitically coupled to a directly fed patch antenna, coupling between the bands is intentionally induced. In the latter case, in which each patch antenna is directly and separately fed, coupling arises from the existence of non-zero surface currents on the radiating patch of the lower patch antenna or antennas at the point or points where the outer conductor of the coaxial feed for the upper patch antenna contacts the radiating patch of the lower patch antenna. As a result, significant effort must be expended in designing circuit componentry to assure adequate isolation between the separate operating bands. Not only is such circuitry difficult to design, but it adds significant expense to the cost of the antenna assembly.
U.S. Pat. No. 5,940,037 owned by the same assignee as the present application, and which is incorporated fully herein by reference, discloses a stacked patch antenna assembly with improved frequency band isolation. Particularly, that patent discloses an exemplary stacked patch antenna assembly in which two patch antennas are fed by separate conductors. A coaxial feed for the upper patch antenna runs through an aperture in the lower patch antenna that is coincident with the null point of the lower patch antenna. The inner conductor electrically couples to the null point of the radiating patch of the uppermost patch antenna. Preferably, the outer conductor of the coaxial feed cable for the upper patch antenna is electrically connected to both the ground plane and the lower patch antenna. The outer conductor of the coaxial feed presents to the radiating patch of the upper antenna an inductance to ground referenced at a ground plane. The lower patch antenna is fed by a separate coaxial conductor that is coupled to a natural feed point of the radiating patch of the lower patch antenna.
With the ever increasing number of mobile communication services available to individuals the number of separate electronic communication devices (either hand held or for use in a motor vehicle) that a person or vehicle must carry is becoming problematic. Such services and devices include cellular telephones, wireless personal digital assistants (PDAs), GPS receivers and pagers. Accordingly, there is a push to integrate electronic communication devices into fewer separate hardware components. Inherent in this trend is a desire to integrate more and more antennas that operate in different frequency bands into an integral antenna assembly that is reasonably compact and effective.
Accordingly, it is an object of the present invention to provide an improved stacked patch antenna assembly.
It is another object of the present invention to provide a stacked patch antenna assembly with improved frequency band isolation.
It is a further object of the present invention to provide a stacked patch antenna assembly with pattern diversity.
The invention is a multiple stacked patch antenna assembly in which the number of possible patch antennas is theoretically unlimited and which provides excellent isolation between the frequency bands. In an exemplary antenna assembly with four antennas, four patch antennas are stacked above a ground plane with the radiating patch of each patch antenna (other than the uppermost antenna) serving a secondary purpose of acting as a ground plane for the patch antenna above it. The aforementioned ground plane serves as the ground plane of the lowest antenna in the stack. A single coaxial cable feeds the two uppermost patch antennas, with the radiating patch of the uppermost patch antenna coupled at its null point to the inner conductor. The upper antenna also may contain an etched transmission line to obtain the xe2x80x9cnatural feed point,xe2x80x9d if other than annular radiation is desired, as discussed in further detail below. The radiating patch of the second uppermost patch antenna is parasitically coupled through the uppermost patch antenna to the feed. The inner conductor of this feed passes through an aperture in the second uppermost patch antenna without making electrical contact therewith. The outer conductor of this feed is coupled to a ground plane and passes through apertures in the third and/or fourth uppermost patch antennas (the two lowest patch antennas). The outer conductor is electrically coupled to one or both of the two lower patch antennas. The apertures in the three lower antennas through which the inner conductor passes are all at null points of the radiating patches.
The outer conductor is grounded to the ground plane. The inner conductor passes through the lowermost patch antenna without electrically contacting it and is electrically connected to a fifty ohm point of the radiating patch of the patch antenna of the second lowest patch antenna. The two lower patch antennas are fed by a separate feed conductor. The upper of the two lower patch antennas (i.e., the second lowest patch antenna) is electrically coupled to the separate feed conductor, while the lowest patch antenna is inductively coupled to the separate feed conductor through the second lowest patch antenna.
The patch antennas preferably are arranged in descending order according to their operating frequency with the highest frequency antenna at the top of the stack and the lowest frequency antenna at the bottom of the stack. Accordingly, each successive patch antenna is larger than the one above it, making it more suitable as a ground plane for the antenna above it.